1. Field of the Invention
The present invention relates to an aqueous ink, an ink set, an ink jet recording method, an ink cartridge, a recording unit, and an ink jet recording method.
2. Description of the Related Art
In recent years, it is demanded to improve the image quality, specifically, the gradation or color uniformity of an image formed on a recording medium, in particular, plain paper by an ink jet recording method. For example, the gradation can be improved by reducing the area of one dot formed from ink on the recording medium. In contrast, when the area of one dot is large, a plurality of dots are apt to link with each other, so it may be unable to strictly control the area of the surface of the recording medium coated with the ink even when the amount of the ink to be applied per unit area is increased or decreased. In addition, a plurality of dots is apt to link with each other, so the blurring of the ink is apt to occur on the recording medium in some cases.
In order to solve such problems, there are proposals that the blurring of ink on a recording medium is suppressed by enlarging the static surface tension (the so-called “surface tension”) of the ink. For example, the proposal that the blurring of ink is suppressed by setting the surface tension of the ink at 20° C. to be 45 mN/m or more, has been made (see Japanese Patent Application Laid-Open No. S63-213581). The proposal that the blurring of ink is suppressed by setting the surface tension of the ink to be 40 mN/m or more, has also been made (see Japanese Patent Application Laid-Open No. 2004-83621).
In addition, a proposal in which attention is focused on the dynamic surface tension of ink has been made to serve as a technology for controlling, for example, the blurring or permeation of the ink on or into a recording medium. For example, the following proposal has been made (see Japanese Patent Application Laid-Open No. H09-296139): the blurring of ink is suppressed by setting the surface tension of the ink in a bubble period T (second/bubble) of 0.2 or less to be 40 mN/m or more, and the ejection reliability of the ink is improved by setting the surface tension of the ink in a bubble period T (second/bubble) of more than 1 to be less than 50 mN/m.
The following proposal has also been made (see Japanese Patent Application Laid-Open No. 2003-231838): the ejection reliability of ink is improved by setting the dry viscosity of the ink to be 100 mPa·s or less, and the dynamic surface tension characteristic of the ink at a specific lifetime is specified. To be specific, it is described in the document that the blurring of the ink is suppressed by setting the dynamic surface tension of the ink at a lifetime of 10 milliseconds to 45 mN/m or more, and the quick-drying property of the ink is improved by setting the dynamic surface tension of the ink at a lifetime of 1,000 milliseconds to be 35 mN/m or less.
Further, a proposal in which attention is focused on the relationship between the dynamic surface tensions of a plurality of inks of which an ink set is composed has been made to serve as a technology for controlling, for example, the blurring or permeation of different colors on plain paper (see Japanese Patent Application Laid-Open No. 2006-63322). That is, in the ink set composed of black, yellow, magenta, and cyan inks, the dynamic surface tensions of the respective inks at a temperature of 25° C. and the same lifetime in the lifetime range of 30 milliseconds to 1,000 milliseconds by a maximum bubble pressure method are specified as follows: (1) the dynamic surface tension of the yellow ink is equal to or larger than that of the black ink, (2) a difference in dynamic surface tension between the yellow ink and the black ink is 5 mN/m or less, (3) a difference in dynamic surface tension between the yellow ink and the magenta ink, and a difference in dynamic surface tension between the yellow ink and the cyan ink are each 3 mN/m or more, and (4) the dynamic surface tension of each of the magenta ink and the cyan ink is lower than that of the black ink. In the document, it is described that those specifications prevent different colors from mixing with each other at a boundary portion between them on plain paper.